I. Field of the Invention
The present invention relates to cellular communications systems and more particularly to methods and an apparatus for changing forward traffic channel power allocation in a code division multiple access (CDMA) cellular communications system.
II. Discussion of the Background
In a CDMA cellular telecommunications system, a common frequency band is typically used for communicating from a mobile to a set of base stations, and another common frequency band is typically used to communicate to the mobile from the set of base stations. In other instances, a common set of frequency bands may be used to conduct communications. A primary benefit of transmitting multiple communications over a common frequency band is an increase in the capacity of the cellular telephone system. The IS-95 standard, promulgated by the Telecommunications Industry Association (TIA), is an example of a highly efficient CDMA over-the-air interface that can be used for implementing a cellular telephone system.
The set of communications conducted over the same bandwidth in a CDMA cellular telecommunications systems are separated and distinguished from one another by modulating and demodulating the data transmitted using pseudo-random noise (PN) codes known to both the receive and transmit systems. The other communications appear as background noise during the processing of any particular communication. Because the other communications appear as background noise, CDMA protocols such as IS-95 often employ extensive transmit power control in order to use the available bandwidth more efficiently. The transmit power control keeps the transmit power of each communication near the minimum necessary in order to conduct communications successfully. Such transmit power control facilitates the processing of any particular communication by reducing the level of background noise generated by the other communications.
Another benefit of having base stations transmit to mobiles on the same frequency band, and of having mobiles transmit to base station on a second frequency band, is that "soft handoff" may be used to transition a mobile from the coverage area of a first base station to the coverage area of a second base station. Soft handoff is the process of simultaneously interfacing a mobile with two or more base stations. Soft handoff can be contrasted with hard handoff during which the interface with the first base station is terminated before the interface with the second base station is established.
As one might expect, soft handoff is generally more robust than hard handoff because as least one connection is maintained at all times. Methods and systems for conducting soft handoff in a CDMA cellular telephone system are disclosed in U.S. Pat. No. 5,101,501, filed Nov. 7, 1989, entitled "Method and System for Providing a Soft Handoff and Communications in a CDMA Cellular Telephone System", and U.S. Pat. No. 5,267,261, entitled "Mobile Station Assisted Soft Handoff in a CDMA Cellular Communication System", both assigned to the assignee of the present invention and incorporated herein by reference.
In accordance with the soft handoff procedure described in the above referenced patents, each base station transmits a respective pilot channel that is used by the mobiles to obtain initial system synchronization and provide robust time, frequency and phase tracking of the cell-site transmitted signals. The pilot channel transmitted by each base station uses a common spreading code (i.e., pseudo-noise sequence) but uses a different code phase offset so that the mobile can distinguish the pilot channels transmitted from the respective base stations.
During a soft handoff, two or more base stations transmit the same forward link data to the mobile. The mobile receives the signals from the set of base stations and combines them. A method and apparatus for performing such combining is described in U.S. Pat. No. 5,109,390, filed Nov. 7, 1989, entitled "Diversity Receiver in a CDMA Cellular Telephone System", assigned to the assignee of the present invention and incorporated herein by reference, discloses a diversity combining method for use in a CDMA cellular telephone system.
While soft handoff provides a more robust connection, in some instances soft handoff also has a negative effect on the overall capacity of the CDMA cellular telephone system. This is because the multiple forward link transmissions generated during a soft handoff may increase the total transmit power used to conduct the corresponding communication. This increased transmit power increases the total background noise generated by the system, which in turn may decrease overall system capacity.
Whether soft handoff increases or decreases system capacity is typically dependent on the environment the mobile is experiencing during soft handoff. If the mobile is experiencing a fading environment, the increased diversity provided by soft handoff is generally beneficial to system performance because the signals generally fade independently. When the mobile is in a non-fading environment, however, the diversity of data source is typically redundant. Therefore, for non-fading environments the benefit provided by increased diversity of signal source typically does not justify the overall increase in transmit power caused by soft handoff.
Thus, the present invention is directed to improving the performance of a CDMA telecommunications system by optimizing the configuration of a CDMA communications system during a soft handoff, in a multicarrier environment, or both in response to the environment in which the communications are being conducted.